How to Find active Tab in Tabbed MDI

I want my Tabbed MDI page to register command bindings to the common windows commands (Save, Edit, etc.) and then call a command on a control in an MDI Container as appropriate, but I don't see how to get a reference to the Active window within the MdiContainer. MainDockSite.ActiveWindow doesn't restrict possible windows to only those in the MDI container. How can I do this?